This invention relates to an apparatus for detecting the peaks of wavelength-division-multiplexed light and an apparatus for controlling this light. More particularly, the invention relates to a peak detection apparatus for detecting the peaks of wavelength-division-multiplexed light and a control apparatus for controlling the intensity of wavelength-division-multiplexed light based upon maximum peak value.
The development of multimedia networks in recent years has been accompanied by increasing demand for information, and much greater capacity and more flexible network formation are being sought for trunk-line optical transmission systems where information is concentrated. At the present time, transmission using wavelength division multiplexing (WDM) is the most effective means of coping with system demand and has been implemented commercially mainly in North America. In such WDM transmission systems, management of the optical level of each channel in the transmission line is important, and the advantages and disadvantages of such management affect greatly the operating state of such functional devices as optical amplifiers, and this in turn changes transmission quality significantly. In WDM transmission, therefore, management of each wavelength level and S/N ratio, etc., is ultimately required in all repeater stages.
However, deploying a level detector or level controller for each wavelength at all repeater stages is inappropriate from the standpoint of reducing the cost of optical transmission systems. There is demand for the minimum necessary functions for detecting and controlling wavelength-division-multiplexed light in a simpler manner that takes cost into consideration. Various measures have been proposed heretofore from this point of view.
FIG. 17 shows an example of an apparatus for detecting wavelength-division-multiplexed light according to the prior art. Here a simple light-spectrum monitor is provided. This is an example reported in a paper (ECOC′ 97, Tu3, p. 147) by K. Otsuka et al. This is a technique through which wavelength-division-multiplexed light emitted from a fiber 101 undergoes wavelength separation by a diffraction grating 102 and impinges upon a photodiode array 103 so that the level of each wavelength is detected by a photodiode. The levels of the wavelengths are detected by the minimum number of photodiodes necessary for point-to-point monitoring.
FIG. 18 illustrates a second example of an apparatus for detecting and controlling wavelength-division-multiplexed light according to the prior art. This example is reported in a paper (IEEE Photon. Tech. Lett., vol. 10, p.734, 1998) by K. Suzuki et al. In this apparatus for detecting and controlling wavelength-division-multiplexed light, first and second optical-fiber amplifiers 110, 120 for control to uniformalize optical gain are cascade-connected, a light attenuator 130 is provided between them, and a feedback circuit 140 is provided in such a manner that the intensity of output light from the second optical-fiber amplifier 120 will be rendered constant.
The first and second optical-fiber amplifiers 110, 120 respectively include rare-earth fibers, e.g., erbium-doped fibers 112, 122, for amplifying wavelength-division-multiplexed light; laser diodes (excitation light sources) 113, 123 for generating excitation light the wavelength of which is shorter than that of signal light and the energy of which is greater, and for introducing this light to the erbium-doped fibers; optical branchers 114, 124; photoreceptors (photodiodes) 115, 125 for detecting the power of the wavelength-division-multiplexed light output by the respective optical-fiber amplifiers; photoreceptors (photodiodes) 116, 126 for detecting the power of the wavelength-division-multiplexed light input to the respective optical-fiber amplifiers; and optical-gain controllers 117, 127 for inputting feedback signals to the excitation light sources 113, 123, respectively, of the respective optical-fiber amplifiers in such a manner that the power ratio (optical gain) of the input light of the respective optical-fiber amplifiers to the output light thereof will become a set gain.
The feedback circuit 140 includes a wavelength demultiplexer 141 for separating the wavelength-division-multiplexed light, which is output by the second optical-fiber amplifier, into individual wavelengths and outputting the same; photodiodes 1421-142n for detecting the intensities (levels) of respective wavelengths λ1-λn; a maximum-value detector 143 for detecting the maximum value from among the levels of the wavelengths; and an optical-output uniformalizing controller 144 for inputting a feedback signal to the light attenuator 130 in such a manner that the optimum value will become the set value. The light attenuator 130 controls the optical level based upon the feedback signal.
In the second example of the prior art, a high output is obtained by cascade-connecting the optical-fiber amplifiers. In an optical-fiber amplifier, gain varies depending upon wavelength, though the gains of the respective wavelengths can be made uniform (the wavelength-dependence of gain can be uniformalized) by performing control to uniformalize gain. Further, since the gains of respective wavelengths can be made uniform, the levels of the respective wavelengths can also be made approximately uniform. As a result, by detecting the wavelength for which power is maximum and performing control in such a manner that this maximum value becomes the set value, it becomes possible to perform control to uniformalize the power of the output light. In other words, it becomes possible to perform control to uniformalize the power of the output light by controlling only one wave of the maximum power irrespective of the number of channels.
The second example of the prior art resembles the first example in that level is detected on a per-wavelength basis. The second example of the prior art differs from the first example in that (1) wavelength-division-multiplexed light is separated into individual wavelengths, using a wavelength demultiplexer such as an arrayed-waveguide grating (AWG), in a state in which the wavelength-division-multiplexed light is enclosed within optical fiber; (2) after the power of each channel (each wave) is detected, the maximum value of these is calculated and fed back to the light attenuator to uniformalize the power per channel; and (3) the number of wavelengths of received light is limited owing to a limitation upon the number of wavelength branches from the wavelength demultiplexer.
FIG. 19 shows a third example of an apparatus for detecting and controlling wavelength-division-multiplexed light according to the prior art. This is described in a report by Saeki, et al. (NEC Giho, vol. 51, no. 4, p. 45, 1998). In this apparatus for detecting and controlling wavelength-division-multiplexed light, a light circulator 150 inputs entrant wavelength-division-multiplexed light to a wavelength demultiplexer 160, which separates the wavelength-division-multiplexed light into wavelengths λ1-λn of respective channels. The thus demultiplexed wavelengths λ1-λn of the respective channels are input to photodiodes 1741-174n via variable light attenuators 171-171n, total reflection mirrors 1721-172n, and optical branching couplers 1731-173n, respectively. The photodiodes 1741-174n photoelectrically convert the input light of the respective wavelengths and feed back the resulting signals to the variable light attenuators 171-171n . As a result, the levels of the respective wavelengths of the respective channels are regulated individually to a fixed level.
The apparatus for detecting wavelength-division-multiplexed light shown in FIG. 17 is structurally large in size and involves an optical system. Consequently, this arrangement is not suited to an apparatus for detecting and controlling wavelength-division-multiplexed light necessary at all repeater stages.
The apparatus for detecting and controlling wavelength-division-multiplexed light according to the second example of the prior art in FIG. 18 requires a costly wavelength demultiplexer. Moreover, there is a limitation upon the number of channels into which wavelength is divided by the wavelength demultiplexer, and the apparatus cannot deal flexibly with changes in the number of channels or changes in wavelength.
The apparatus for detecting and controlling wavelength-division-multiplexed light according to the third example of the prior art in FIG. 19 performs level adjustment individually for each channel and therefore makes possible control that is highly precise. However, the costly wavelength demultiplexer is required. Moreover, a light attenuator, total reflection mirror, optical brancher and photodiode are required for each channel, as a result of which the apparatus takes on a large size. In addition, changes in the number of channels or changes in wavelength cannot be dealt with in a flexible manner.
Thus, as described above, a wide variety of methods and arrangements have been proposed for detecting and controlling wavelength-division-multiplexed light, but each of these proposals involves problems. The functions that are required of an apparatus for detecting and controlling wavelength-division-multiplexed light are as follows:                It should be possible to detect optical power per wave of light whose wavelengths are allocated to respective channels, and to perform control to uniformalize optical power (or to detect the maximum value of optical power and perform control to uniformalize the same.        It should be possible to ascertain the number of multiplexed wavelengths.        The arrangement should be independent of channel wavelength and number of multiplexed wavelengths.        The apparatus should be low in cost, small in size and simple in structure.        
The conventional examples of apparatus meet some but not all of these requirements.